Pointing devices for use in computers, and the like, are well known. These devices are often referred to as mice. In general, a mouse is a handheld device that is moved by the user over a surface to control the movement of a cursor on a display screen that is connected to the mouse. Typically, the cursor is moved in a direction and by an amount determined by the amount that the mouse moved over the surface. The original mice sensed the motion of a handheld device over a surface using mechanical means such as a ball that rolled against a set of rollers as the mouse was moved over the surface. However, this type of mechanical system suffered from a number of problems such as the accumulation of grease and other debris on the rollers as a result of material picked up from the surface by the ball.
These problems have been overcome by mice based on optical sensing. An optical mouse typically includes a light source that illuminates the surface under the device at an oblique angle and a camera that takes a series of pictures of the illuminated surface. The light source typically includes a light-emitting diode (LED) and a lens for directing the light to the appropriate area of the surface at an angle that enhances any surface roughness. By comparing successive pictures, the mouse controller can determine the distance and direction that the mouse moved between the two pictures. This comparison typically involves shifting one of the images relative to the other and then computing some measurement of the correlation of the two images. The shift that provides the maximum correlation is then assumed to be the amount of displacement of the mouse between the two pictures.
These devices work well on surfaces that have a texture that gives rise to images that can easily be compared to one another by shifting one of the images. However, many surfaces are less than ideal. For example, a conventional optical mouse moving over a glass surface performs poorly, if at all, either because the surface is too smooth to provide the desired image or because the surface is transparent, and the underlying surface is out of focus. Other surfaces provide tracking information that has insufficient precision for many applications.
To provide optical mice that perform better on these less than ideal surfaces, optical mice having a laser diode in place of the LED have been developed. When the surface is illuminated with coherent light from the laser, the resulting images are enhanced due to the interference of the coherent light reflected from the surface. Optical mice based on a number of such interference effects have been suggested.
While these coherent light mice provide improvement over the LED-based mice, they do so at an increased cost. Hence, manufacturers of mice tend to offer a range of products having different levels of performance and different costs. The mice can differ in the light source, the image sensor, or the complexity of the control chip that controls the image sensor and computes the movement of the mouse over the surface. As a result, the manufacturer must stock parts for many mouse designs. The introduction of laser-based mice has further increased the number of parts that must be maintained in inventory. In general, the mouse includes a controller chip that sets the current that flows through the light source and performs the various calculations needed to compute a displacement. LED light sources require different driver circuits than laser diode light sources. Hence, the manufacturer must stock both new controller chips and lasers in addition to the parts utilized in the assembly of LED-based mice. In addition, the design time associated with developing a new optical mouse having a different light source from those currently used is increased by the need to provide a new control chip for the new light source.
In principle, the driver circuitry could be located on a separate chip so that the same controller chip could be used for both mice based on LEDs and lasers. However, this solution increases the cost of both types of mice. Such cost issues are particularly important in low-cost mice.